Recuperative heat exchangers having flattened tubes that conduct a cool air stream to be heated in heat exchange relation with a hot gas stream flowing over the tubes, with the intent of transferring heat from the hot gas to the cooler air to be heated, are well known in the art. To achieve maximum effectiveness in such apparatus it is essential that a maximum amount of heat be transferred from the hot gas to the cooler air before the heated air stream is exhausted to its place of use. Moreover, it is important that such a heat exchanger be capable of continuous operation at highly elevated temperatures in corrosive atmospheres.
Heat exchangers used for operation in the above outlined capacity are consequently limited in effectiveness by the material from which they are made and by the extreme temperatures to which they are subjected. Thus a heat exchanger tube subjected to continuous operation at 1200.degree. F. would usually have a much shorter life expectancy than would a tube operating at 600.degree. F.